Synthesis of quantum dots (QDs) with widely size-tunable optical absorption and high photoluminescence quantum yield (PL QY) via a facile route is highly desired. By introducing tributylphosphine (TBP) into a relatively green synthesis method based on the use of S, PbCl₂ and oleylamine (OLA), we conveniently synthesized ultrasmall PbS QDs with the first excitonic absorption peak wavelength as short as 705 nm, without using a glove box, which cannot be achieved by previously reported approaches, without involving smelly S precursors (such as bis(trimethylsilyl) sulfide). Such synthesized PbS QDs show narrow size distributions without any aggregation and demonstrate high PL QY in the range of 60–90%, depending on the QD size. Based on nuclear magnetic resonance spectroscopy and X-ray diffraction investigations, TBP was found to act as the passivation ligand on the surface of QDs while simultaneously assisting the transformation of PbCl₂–OLA into more reactive Pb(OH)Cl that can directly participate the nucleation process, yielding ultrasmall PbS QDs. This new finding renders Pb(OH)Cl a very promising, new lead precursor for convenient synthesis of PbS and other lead-based QDs. We also demonstrate that the process can be readily scaled up. After synthesizing a thin CdS shell (∼0.1 nm), ultrasmall core/shell QDs with a large Stokes shift (0.36 eV) and good stability were employed for fabricating near infrared (NIR) luminescent solar concentrators, which led to a record-high optical efficiency of ∼1.2% at a geometric factor of ∼50 (10 cm in length). The TBP route developed herein is very promising for synthesizing high quality ultrasmall QDs that have high potential in NIR-related applications.

Graphene quantum dots (GQDs) are promising for bioimaging applications due to their unique optical properties and good biocompatibility. However, the facile synthesis of GQDs with long-wavelength emission and high quantum yield (QY) under long-wavelength excitation is still a challenge. Herein, dual-emissive GQDs were synthesized via a facile one-pot solvothermal method using 2,7-dihydroxynaphthalene (2,7-DHN) and piperazine anhydrous (PA) as the raw materials. The obtained GQDs exhibited a green-emission band (core state emission) and a red-emission band (surface state emission), which originated from the inner π-conjugated structure and various surface functional groups, respectively. The π-conjugated structure mainly comes from the aromatic rings in 2,7-DHN, and the N–H of PA acts as the main nitrogen source, which is beneficial for the QY. The obtained GQDs showed obvious concentration-dependent optical properties. Importantly, they are able to generate red emission with a high absolute QY of 54.9% in ethanol and 24.9% in water, among the best values of reported GQDs. In addition, the prepared GQDs have good water-solubility, low cytotoxicity, and superior stability against photobleaching. Based on the above advantages, the as-prepared GQDs were successfully applied for labeling HeLa cells in vitro. Therefore, our prepared GQDs demonstrated that they can be an efficient biomarker for fluorescent bioimaging.